Electrodeposition of chromium and baths therefor



Patented Apr. l2, 1938 PATENT OFFICE unc'rnonarosrrron or cnnormm Ann Barns 'rnnnnroa Oskar Kramer, Leipzig, Germany, assignor Theodor Haebler, New York, N. Y.

\ No Drawing. Application March 10,1933, Serial r No. 661,087, In Germany October 4,1932

9 Claims. (Cl. 204-1) This invention relates to the electrodeposition of chromium and one object of the invention is to prepare or establish a bath of such composition and constitution that improved results both as to excellence of product and as to economy of operation are obtained.

Heretofore the general commercial practice has been to obtain a deposit of metallic chromium from electrolytes which consist of more or less concentrated aqueous chromic acid solution containing percentages, within definitely; prescribed limits, of acid radicals without the presence 01 which within such limits it has thus far been deemed to be impossible to obtain metalli c chr0- mium on a commercial scale by electrolysis from an aqueous chromic oxid solution.

The present invention is designed to deposit chromium without any acldradical content in the'bath other than chromic acid and instead 01' relying upon the presence of an acid radical such as the sulphate radical or any other, the new bath is such that there will be no suchcatalytic anion present.

'I'he current efliciency .of chromium-plating processes thus far used lags materially behind the current efliciency of other galvanic baths,

such for example as are used in nickel, copper, silver, etc., plating, the greater part of the electrical energy being consumed by the development of hydrogen. The loading oi. the chromium deposit with occluded hydrogen is so high that fissures develop in the deposit as the result of the tensions within the chromium. The range of current density within which the chromium deposits in bright or easily polishable form, is narrow. As already stated, the theory which has thus ;far prevailed with respect to the depositionof chromium from aqueous chromic acid solutions is that the chromium separation is possible only in the presence of the following acid radicals: sul

with the chromic acid or. otherwisel complex 0:0. (anhydrous chromic acid) gr Cn(0r0)a,(chromium chromate) 5 gr. NaOH sodium hydroxid) 3 gr. KHC; 40a (potassium ,bitartratc) The throwing power developed in the known chromo and/or chromi combinations whose complex portion is a cation If in addition hereto there is added to the electrolyte complex-forming combinations oi organic or inorganic nature, the eiiect will be materially increased, provided al- 5 ways that these complexes are present as cations. The presence of acid radicals is absolutely unnecessary but if for any reason they should be present they will in no way reduce the efliciency oi. the metals above referred to which are more 10 electropositive than chromium or of the complex cations. To demonstrate the accuracy of the foregoing facts a Supp y of the purest'commercial chromic acid (still containing traces of about .2%. sulphuric acid) was treated with bariumhy- 15 droxid to eliminate the sulphate content. This chromic acid which was then entirely free of sulphate was used in the subsequent bath with the results set forth in this specification. Similar baths in which the treatment of the chromic acid 0' with barium hydroxid was omitted showed no departure from the results obtained in the first. series of tests, thereby demonstrating that the efiects of such traces of sulphuric acid as are still 400 gr. CIOI lliter HQQ g: fggg 12 gr. xrrcimou In the above formulae the reference to chromium chromate is to the commercial form of this material which is not wholly chromium chromate but contains appreciable amounts of other chromium 4 compounds, none of which, however, except by 40 way of mere traces, supplying any acid radical to the bath.

In the above baths the sodium or potassium or both can be replacedby aluminum or other metals more electronegative than chromium,

The bitartrate content can be increased up to about gr. per liter without appreciably affecting the efliciency of the operations." Instead of the sodium hydroxid, cadmium oxid, cadmium carbonate, nickel carbonate, cobalt carbonate,.: V etc., canbe added. These cations can also be introduced into the bath by electrolytic solution at the anode. If cadmium and cobalt are used'as ]per liter cations it is still possible to chromium plate withu out the presence of the tartrate though with diminshed efliciency.

The metals are introduced or added to the bath in the form of oxids, hydroxids, or carbonates in order that there may not be introduced any foreign acid radical and for the same reason-it is advisable to' add the metals by electrolytic solution at the anode.

The function of the bitartrate is to form a complex cation as hereinafter described. Any

quantity of bitartrate that does not go to the formation of this complex chromium cation is oxidized to CO2 and H20 by the chromic acid in .the' bath and is thus eliminated. It will be understood, of course, that in the place of bitartrate other complex-forming combinations such as glucose, cane sugar, or fructose may be used as well as such bivalent acids derived therefrom as are completely destroyed as acids by oxidation in the chromic acid electrolyte (as 'in the case of the bitartrate above described). Experiments with other organic acids of the structure:

' CO2H(CHOH) nCQzH wherein 1:. equals 2, 3, 4 indicate that they,

too, behave in a similar manner and similarly give satisfactory results. w

The general formula of a typical complex may be represented as follows:

M being a'metal (Ac) a radical of a complex-forming acid, such as tartaric, acetic, or formic, etc.

R is a constituent of an acid of chromium,

a: plus 1/ equals 3 or 4, considering valence 11. plus 0 equals 8 1: equals 1 to 3. v

As an example of such a complex, one analyzed byR. Weinland and E. 'Gussmann is given below:

In this complex, potassium, cadmium, cobalt,

,or nickel may replace the iron; in a manner, of course, considering valence.

even at the extremely low current density of less than 1 ampere per squaredecimeter to cause the deposit of bright metallic chromium. Up to now the lower limit at which deposit of metallic chromium was possible at all was a current density of 2 amperes per square decimeter.

A very important feature of the new process is' the greatly increased throwing power which is industrially exceedingly important.

' The addition of the metals above referred to and of the complex-forming substances may vary within wide limits. A fairly satisfactory electrolysis already begins when there is a very small content of the cations referred to and the perform-a ance of. the bath remainsexcellent though the a cation percentage increases to as much as 20%+ (with relation to the chromium trioxid contained inthe electr lyte).

It will be observed that in the described new process a method becomes available wherein chrome plating is made independent of the presence of acid radicals in the bath and whereby satisfactory chrome plating may be accomplished in a bath in which acid radicals are permitted to be present only as traces (existing through impurities) and materially less than 0.4% of the sulphate radicals (or their equivalent) with respect to the chromic acid.

When I refer in the claims to a complex organic chromium compound I refer to the chemical compound which remains in a chromium plating bath with the chromic acid after the addition to the chromic acid solution of chromium chromate, sodium hydroxid, and potassium bitartrate or their respective equivalents. material to a strong chromic acid solution results in its destruction, .but when an organic portion of the added organic material is still preserved in such a bath after the destructive action of the chromic acid has spent its force, the chemical The addition of organic body which remains exists in .the bath in theform of what I term a complexflto wit, a non-simple cation which has in its make-up an-organic por-. tion which, if not held by other components which are joined with it, would be destroyed by the chromic'acid.

I- claim:

. 1. An electrolytic bath for the electrodeposition of chromium comprising chromic acid and a reaction product of chromic acid, a chromium salt, analkali metal hydroxide, and an alkali metal salt of an organic acid oxidizable by chromic acid, said bath being substantially devoid of acid radicals other than chromic acid.

2. An electrolytic bath for the electrodeposition of chromium, comprising the reaction product of an excess of chromic acid, chromium chromate, sodium hydroxide and potassium bitartrate, said bath being substantially devoid of acid radicals other than chromic acid. v

3. The'method of electrodepositing chromium from solutions of chromic acid which comprises establishing a bath containing chromic acid as its major component and an agent which will enable the bath to become available for chrome plating, said agent being a complex cation which remains stable in the presence of the excess of chromic acid, passing an electric current through the bath from an anode to an object cathode and maintaining in-said bath a condition where it initially is and continues throughout the period of its use to be substantially devoid of acid radicals other than chromic acid.

4. The method set forth in claim 3 in which the complex cationis formedby causing areaction between chromic acid, an organic substance, and material participating in the reaction and containing a'metal more electronegative than chromium, and by such reaction causing the formation of a complex compound which includes chromium, a metal more electronegative than chromium, and an organic portion.

' 5. The method set forth in claim 3 in which the complex cation is. formed in the bath by causing a reaction therein between its chromic acid, an added organic substance, and addedmaterial participating in the reaction and con taining a. metal more electronegative than chromium, and by such reaction causing the formation within the bath of a complex compound which includes chromium, a metal more electronegative than chromium, and an organic portion. 6. An electrolytic bath for the electrodeposition of chromium comprising chromic acid as its major component and an agent which will enable the bath to become available for chrome plating, said agent being a complex'cation stable in the presence' oi the chromic acid in the bath, said bath being initially and continuously throughout its use substantially free from acid radicals other than chromic acid. v

7. An electrolytic bath such as described in claim 6 in which the complex cation is a complex organic chromium compound.

8. An electrolytic bath such as described in claim 6 in which the complex cation includes' chromium, a vmetal more electronegative than pound and a compound of a metal more electronegative than chromium, osKAa- KRAMER. 

